GPS versus RNAV

[Bman.]

Evening all-

I am having a difficult time understanding GPS versus RNAV. I know that RNAV is area navigation. I have heard that GPS alone can not tackle RNAV and that other things are needed to do area navigation.

I have read this site: http://www.allstar.fiu.edu/aero/RNAV.htm

It's seems to me that GPS/RNAV are nearly one in the same in terms of navigation. I see approaches that are labeled RNAV(GPS). If your equipment meets the TSO C-129 certification then you can fly the approach.

I guess my confusion lies in that I read and see charts for RNAV where they talking about "moving a VOR" or other navaid to a location of choosing. Seems to me when I set up a flight plan (simulator) that all of my waypoint are intersections on victor ways etc. I don't see much RNAV in this..

Does anyone have an "easy' explanation between GPS nav versus RNAV?

Much appreciated- and don't get me started on LNAV and VNAV. I have a grasp on those I suppose (but is LNAV more precise than GPS?...how come GPS can't do LNAV?) - does it all come down to on board equipment? I am using FSX (Garmin 500)

The questions go on and on.

Thanks all

Benjamin

[Dugie8]

You basically have it. RNAV and GPS are the same thing. The old (god it hasn't been that long) RNAV units where basic trig calculators, you could enter in a VOR freq a radial and distance and fly to that point just as if it was a VOR (radials to from). There were/are?? approaches that were RNAV approaches that you set up these waypoints based off a nearby VOR. I flew some PA28s that had these units, worked great, there were/are?? charts designed for using those RNAV units. All the VORs had bold lines for N, E, S and W and you simply drew your course and entered in waypoints as they crossed those cardinal radials.

Now, with the advent of GPS and how widespread it is, GPS has become RNAV on the approach plates, instead of GPS RWY 17.

[BobDDuck]

GPS is a form of RNAV. Area navigation is pretty much "point to point" navigation meaning you don't need to go from ground station (like a VOR or NDB) to ground station but rather can go in a straight line. RNAV can be made up of a combination of GPS, IRS, VOR/VOR, DME/DME, VOR/DME and a whole bunch of other stuff. Basically an FMS (or straight GPS) just takes a bunch of readings using these various methods and then averages them out and uses that as your position.

There are certain levels of RNAV accuracy. Different airspace and different maneuvers require different levels of accuracy. They are classified as RNP or "required navigational performance". Depending on what methods of position fixing your system is capable of, your RNP varies.

LNAV is simply Lateral Navigation. Most autopilots can do this. Tracking a course path (either VOR radials or GPS/FMS course data) to a fix.

[Ian J]

Originally Posted by Dugie8

You basically have it. RNAV and GPS are the same thing. The old (god it hasn't been that long) RNAV units where basic trig calculators, you could enter in a VOR freq a radial and distance and fly to that point just as if it was a VOR (radials to from). There were/are?? approaches that were RNAV approaches that you set up these waypoints based off a nearby VOR. I flew some PA28s that had these units, worked great, there were/are?? charts designed for using those RNAV units. All the VORs had bold lines for N, E, S and W and you simply drew your course and entered in waypoints as they crossed those cardinal radials.

Now, with the advent of GPS and how widespread it is, GPS has become RNAV on the approach plates, instead of GPS RWY 17.

I fly RNAV planes all the time - you're right, they work great. It's getting to the point now where a VOR freq, a radial, and a distance might as well spell out the fix in letters to me.

[Bman.]

Ok- "GPS is a form of RNAV" That's what I was after. It's not GPS or RNAV is one or the other but that you can use GPS and fly some form of RNAV. When flying en-route and going from intersection to intersection, is this considered a form of RNAV in any way or are you just flying via GPS navigation?

Are there specific degrees of accuracy associated with LNAV and VNAV? I was under the impression that LNAV took special equipment - not the simple AP from a Cessna.

Benjamin

[E6BAV8R]

When flying en-route and going from intersection to intersection, is this considered a form of RNAV in any way or are you just flying via GPS navigation?

Yes, it is. GPS is just a much simpler form of RNAV as I understand it. RNAV has many things that it can use to draw up different waypoints while GPS just uses waypoints to go direct to each of these points.

Correct me if I am wrong.

[SIUav8er]

GPS is RNAV, RNAV isnt necissarilly GPS. Basically RNAV is any form of navigation that is not ground based (even though it might USE ground based navaids to fix its position)

The original RNAV systems used VOR and DME to triangulate its position by determining the radial and DME from a certain station.

LORAN and OMEGA (thing of the past) are other forms of "RNAV" systems. I would also assume INS is RNAV as well although I could be wrong.

The whole LNAV, VNAV, LPV thing has to do with GPS WAAS(wide area augmentation system). In a nutshell, heres what it means:
LNAV is your basic IFR certified GPS that provides "lateral guidance" only. No vertical nav (glide slope).
VNAV is when the GPS uses WAAS to calculate a glide slope to follow so you wont have to "dive and drive" to the MDA, basically it just calculates a descent path, then displays it as a glide slope on a non precision approach.
LPV is localizer with precision vertical guidance. Basically it uses the WAAS signal to create an approach that is just as accurate as an ILS. In fact, LPV uses DA instead of MDA, so it is considered a precision approach.


Im not sure why theyre renaming GPS approaches RNAV approaches. Most of them require GPS to fly. im sure someone will chime in with better information here. Im too lazy to look this stuff up right now.

Hope that helped

[ghogue]

Originally Posted by SIUav8er

VNAV is when the GPS uses WAAS to calculate a glide slope to follow so you wont have to "dive and drive" to the MDA, basically it just calculates a descent path, then displays it as a glide slope on a non precision approach.

Almost. VNAV is Baro-VNAV or WAAS.

[SIUav8er]

Originally Posted by ghogue

Almost. VNAV is Baro-VNAV or WAAS.

Right, but LPV and LNAV also uses WAAS (if the reciever is capable of doing so). I was explaining the difference between them

[ghogue]

Originally Posted by SIUav8er

Right, but LPV and LNAV also uses WAAS (if the reciever is capable of doing so). I was explaining the difference between them

LPV requires WAAS. LNAV and LNAV-VNAV do not require WAAS. A WAAS equipped aircraft is capable of doing all three, but it isn't necessary to have except to fly LPV.

Your statement "VNAV is when the GPS uses WAAS to calculate a glide slope" gives the impression that WAAS is required for VNAV and left out the equipment (Baro_VNAV) that VNAV was designed for.